1. Field of the Invention
The present invention relates to a system to measure the condition of a vehicle brake and, more specifically, to a system to detect and generate an electrical signal when the brake actuator travel exceeds a predetermined limit thereby indicating the need for brake service.
2. Description of the Prior Art
The mechanical adjustment of air brakes on trucks and other highway vehicles is a constant source of concern to vehicle owners, drivers and public officials whose duty it is to inspect and to check the road worthiness of such vehicles. In the case of air brakes, various devices have been designed to assist and to expedite the process; some of which are very simple, others of which tend to be relatively complex and, in all likelihood, relatively costly.
A common type of air brake system for trucks utilizes an actuator associated with each brake. The actuator has an opening in one side through which an actuator rod extends, the actuator rod being in turn connected to the actual vehicle brake through a suitable mechanical linkage such as an automatic slack adjuster that is well known in the art. When the vehicle brakes are not being applied, a spring or the like within the actuator holds the push rod in a relatively retracted position when the brake is off. However, when an operator applies the brakes, pressurized air is supplied to the actuator and the pressurized air causes the actuator rod to move along its longitudinal axis, in a direction out of the actuator. This movement continues until the brake engages and provides a return force that balances the force provided by the air pressure.
As the brake wears, the push rod must move through a larger and larger distance from its retracted position to an extended position in which the brake is applied. An automatic slack adjuster takes care of a certain amount of linkage adjustments due to wear but if the process continues uncorrected, the actuator rod travel will gradually reach its upper limit, determined by the geometry of the actuator. When this occurs, the brakes will not be effective even when fully applied by the vehicle operator. Because of this, in the U.S., trucking regulations place an upper limit of two inches on the maximum amount of actuator rod travel between its retracted and extended positions.
Two basic types of solutions have been provided to address the problem of wear in air brake systems. The first of these solutions is a device known as a slack adjuster that is included in the linkage between the actuator rod and the brake. A slack adjuster removes slack from this linkage, thereby reducing the amount of actuator rod travel necessary before the brakes are applied. Both automatic and manual slack adjusters are well known and in use. In an automatic slack adjuster, the slack removal operation is automatic, and occurs whenever a predetermined degree of slack develops in the linkage. However, an automatic slack adjuster is a relatively expensive and complex device, and is subject to occasional malfunction. A manual slack adjuster must be adjusted manually whenever excessive actuator rod travel develops in the brake system. However in general, an operator has no means of readily determining when such wear exists.
In order to overcome the limitations of existing slack adjusters or for use with manual adjustment systems, a variety of brake wear indicators have been devised to provide an indication to an operator that excess actuator rod travel has developed. For example, it is known to provide a brake wear indicator that comprises a sleeve that extends around the actuator rod and through the opening in the actuator through which the actuator rod extends. A relatively loose fit is provided between the indicator and the actuator rod, and a comparatively snug fit is provided between the indicator and the actuator. When the brakes are applied, the indicator is forced out of the actuator along with the actuator rod. When the brakes are subsequently released, the fit between the indicator and the actuator prevents the indicator from retracting. Thus at any given time, visual inspection of the indicator will indicate the maximum travel of the actuator rod since the indicator was last reset into its retracted position. While effective in principle, the prior art brake wear indicator suffers from the problem that there is considerable variation in the size of the opening in the side of the actuator through which the actuator rod extends. Thus, in practice, indicators of various outside diameters must be provided in order to cover the different brands and models of actuators currently in use. In addition, no electronic signal is generated that can be used to alert the operation or the need for service without a visual inspection of each brake assembly.
One of the simplest kind of devices is a split ring plastic sleeve designed to slip over the air pressure chamber actuator rod, and thereafter provide a visual indication of actuator rod travel distance or stroke extension. Excessive actuator rod travel indicates a level of brake lining wear that warrants replacement of the linings. Another device, which likewise acts as a visual indicator, involves modification to the actuator rod itself. A colored band is painted into a groove circumferentially machined in the actuator rod.
Another conventional brake wear indicator comprises an extension of the return spring of the actuating cylinder that extends from the interior of the actuating cylinder to the exterior where it is visible for inspection. When the brakes are applied, the return spring is compressed and the extension projects from the actuating cylinder. The greater the distance the extension projects the greater the wear of the brake linings.
The foregoing and other visual indicators are disadvantageous by reason of their dependence on vision. The ability to see can be hampered by darkness, or by dirt, mud or slush picked up form the road. Further, it can be an awkward and sometimes messy job to move into a position that enables a visual check to be made. Also, more importantly, no warning is given to the operator in the cab so that immediate service can be obtained.
Prior art slack adjusters have used switching devices to signal when the brake adjuster has reached the end of its available travel thereby signaling the need for brake service. The switch, which is mounted to the slack adjuster, is electrically connected to a monitoring device using two connecting wires. These types of devices function to electrically signal when the brake slack adjuster has traveled to its service limit thereby notifying the operator of the need for brake service.
Another method of signaling when brake service is required is a system to monitor the travel of the pushrod of the brake actuator. In one such device, a magnet is secured to the actuator rod of a brake actuator which magnetically activates a switch mounted to the housing of the brake actuator when the rod travels to its service limit. The switch is connected to an electronic circuit to alert the operator of the need for brake service.
These and other prior art systems have the limitations of overly complicated structures that must be supported and are prone to failure. In the case of visual systems, no signal is directly available to the driver while operating the vehicle which could result in an overworn brake condition prior to a service inspection.
The present invention provides an apparatus for electronically signaling when excessive brake wear has occurred in a brake of the type which uses a moveable actuator rod to displace an actuator linkage to activate the brake. A switch is mounted to the actuator housing so as to engage a specific increased diameter section of the actuator rod when the travel of the rod approaches or exceeds a predetermined service limit although the switch can be of the type known in the art as xe2x80x9cnormally closedxe2x80x9d or conversely of the type known as xe2x80x9cnormally openxe2x80x9d depending on the nature of the signal to be provided to the monitor electronics. In the preferred embodiment, the switch is normally closed by action of a molded body and a spring member which encircle the actuator rod although the switch is activated by the action of the body/spring which hold the switch in an activated position when the actuator travel is within prescribed operational limits. When the spring contacts a larger diameter section of the rod, the spring and molded body are moved to open the switch to signal when the actuator rod has reached a service limit position and the brakes need replacement or further adjustment.
According to the present invention, a pair of contacts is mounted within an open end of a C-shaped spring member which encircles the smaller diameter section of the actuator rod. An overmolded body protects the wires and electrical connections, and the switch contacts can be environmentally sealed using a rubber boot member. As the spring member encounters the larger diameter section of the actuator rod, the contacts are pulled apart thereby electrically opening a circuit. In the preferred embodiment, the switch contacts are mounted in a switch assembly on the open end of the C-shaped spring member and C-shaped molded body, and the spring member and overmolded body are mounted to the actuator housing to encircle the actuator rod.
One provision of the present invention is to provide a switching device to electrically indicate when a brake actuator has reached a predetermined travel limit.
Another provision of the present invention is to provide a switching device mounted to the outside of the actuator housing to electrically indicate when a brake actuator has reached a predetermined travel limit.
Another provision of the present invention is to provide a switching device actuated by a change in the diameter of the actuator rod of a brake actuator when the brake actuator has reached a predetermined travel limit.
Another provision of the present invention is to provide a switching device mounted to an open end of a C-shaped spring member having electrical contacts which extend to make electrical contact with each other when a brake actuator is within its travel limits.
Another provision of the present invention is to provide a spring member mounted to the outside of a brake actuator housing which encircles the actuator rod where the spring member is forced apart by a larger diameter section of the actuator rod when a predetermined travel limit position is reached.
The switching system of the present invention requires no additional assembly after the overmolding process except for the addition of a grease boot provided around the electric contacts. The molding process of the invention eliminates several loose parts provided in the prior art design; the loose parts of the prior art design are small, difficult to handle, require assembly and lacked the robust qualities of the overmolded body provided by the present invention. The overmolded material provides consistent location of the spring and contacts and protects the wires. The C-shaped spring member provides a positive load to the contacts in order to maintain a closed circuit.
The macro-composite design provides a steel spring interior diameter and a polycarbonate outer shape that work in concert to provide load to the contact points. The molded material assists in maintaining a load during cold temperatures when the steel spring relaxes and the molded material becomes stiff. Conversely, during high temperatures when the molded material relaxes the steel spring tends to coil tighter.
The C-shaped spring provides a wear surface for the air chamber push rod and protects the wires as the push rod passes back and forth through the center of the switch during normal operation. In the preferred design, the spring is formed with small holes around its circumference, and during the molding process molding material flows into these holes; thus providing a mechanical interlock between the spring and the molding material.
The C-shaped spring member is formed with a pair of tabs at each end at the open end of the C-shaped spring member, and the tabs straddle the wires that are crimped to the brass contact points. These tabs provide a mechanical interlock between the molding material and the spring and help prevent creep of the molding material to insure that the load of the spring is concentrated on the contact points.
The boot seal provided about the contact points provide a physical barrier to prevent foreign objects from violating the surface to surface engagement of the contacts. In addition, the boot provides a means to contain a non-conductive compound which further protects against corrosion and intrusion of dirt and moisture between the contacts.
The molded polycarbonate body allows the switch to be sandwiched between the air chamber and the retainer plate without corrosion.
These and other advantages will become more apparent with reference to the following description and associated drawings.